Flue-gas desulfurization wastewater from coal fired power plants and wastewater from factories refining nonferrous metals contain selenium in some cases. The wastewater rarely has a high selenium concentration. However, even if the selenium concentration is about several milligrams per liter, selenium needs to be removed from the wastewater for environment conservation.
Hitherto, a method of using divalent iron, a method of using metallic iron, a method of using an aluminum salt or metallic aluminum, and the like have been known as a method for removing selenium from selenium-containing wastewater.
The method of using divalent iron include adding a salt of an acid and divalent iron to selenium-containing wastewater, performing a reaction by adjusting the pH to 8.5 to 10, and then performing solid-liquid separation (for example, Patent Document 1).
The method of using metallic iron include adjusting the pH of selenium-containing wastewater to 5 or less, bringing the wastewater into contact with metallic iron to reduce selenium, performing coagulation treatment, and performing solid-liquid separation (Patent Document 2).
The method of using metallic aluminum include adjusting the pH of selenium-containing wastewater to 6 or less to allow copper ions or iron ions to be present, adding a metal, such as metallic iron or metallic aluminum, to adjust ORP to −350 mV or less, and then adjusting the pH to 8 to 10 to perform solid-liquid separation (Patent Document 3).
In the method of using divalent iron or metallic iron, however, separated sludge is colored. This limits the disposal of the sludge. In the method of using metallic aluminum, even when wastewater is adjusted to be acidic and then brought into contact with metallic aluminum, aluminum is not easily leached. Thus, a desired reduction effect is not achieved.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 6-79286
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 9-187778
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 8-224585